There is a disk array apparatus as one of storage devices. A disk array apparatus has a structure in which a plurality of hard disks are connected in parallel, thereby, the disk array stores a great deal of data. Such disk array apparatus has a built-in cache memory and can reduce a time to access data by using the cache memory.
Generally, the disk array apparatus operates by a commercial power supply. For this reason, in the disk array apparatus, when the commercial power supply is cut off, data which has been stored in a cache memory disappears. Taking this into consideration, the disk array apparatus has a secondary battery (battery pack) as a standby power source (auxiliary power supply). In addition, the disk array apparatus is equipped with a function to, in case of a blackout, write data of the cache memory into the hard disk using electric power of the standby power source. As a result, the disk array apparatus prevents a situation in which data in the cache memory disappears due to the blackout.
In Patent document 1 (Japanese Patent Application Laid-Open No. 2006-209218), there is shown an example of a battery unit included in a disk array apparatus. In this battery unit, a battery and a battery charging and discharging circuit for the battery are connected in an one-to-one relationship.
In Patent document 2 (Japanese Patent Application Laid-Open No. 1997-121461), there is shown a battery power supply made by combining a secondary battery and a solar cell. The battery power supply indicated in Patent document 2 has at least two groups of secondary batteries. Further, the battery power supply has a function to switch a group of secondary batteries which have been charged by the solar cell and the group of secondary batteries of a discharged state alternately.
In Patent document 3 (Japanese Patent Application Laid-Open No. 2001-23588), there is indicated a battery pack mounted on a mobile phone. The battery pack is equipped with a function to reduce an amount of charge to about 50%, for example, of full charge when it is in a state of being removed from the mobile phone (in other words, when detection of entering a state of preservation is made).